1. Field of the Invention
The present invention relates to a stabilzer and method for maintaining a drill string on a desired course during drilling operations to drill a well bore.
2. Description of the Prior Art
During the process of drilling a well there are constant, yet variable, forces applied to the drill bit and the drill string assembly. As the bit penetrates the formations being drilled these forces may induce deviation of the drill string from a desired course while drilling either a so-called "straight hole" (vertical or horizontal) or a deviated well bore. A few of the forces applied are from such sources as the dip of the formation being drilled or bored; the weight applied on the drill bit during penetration; the revolutions per minute of the drilling bit while penetrating the formation; the types of changing formation being penetrated; the angle of the well bore being drilled, and the like.
In vertical drilling operations, the force of gravity on a drill bit and drill string acts favorably in producing a vertical hole, however, the differences in formation make up and consistency along with the dynamic forces of the bit and drill string can produce significant intentional or unintentional deviations.
One method that is used to control the hole angle has been the technique of using the "pendulum" principle, that is, the proper placement of stabilizers in the lower drill collar assembly to provide fulcrum points so that the hole angle of the bore hole from the vertical may be increased or decreased depending on the correction required. Once the hole angle has been established it is common practice to "pack" the hole or the lower drilling assembly with stabilizers so as to maintain hole angle and prevents lateral movement of the bit. A common type of stabilizer which has been used is a fixed blade stabilizer that rotates with the drill string. Fixed blade stabilizers that rotate with the drill string tend to drill away the hole wall, thus permitting the drill bit to move laterally and off of the desired bit course.
In addition to hole angle deviation from a vertical standpoint, bore holes drilled with ordinary earth drilling tools may deviate with respect to a fixed reference generally taken as "north or south" with respect to the earth poles. Intentional or unintentional polar deviation in conjunction with the above mentioned vertical deviation, constitute the art, science and craftmanship of "directional drilling". Once the art, science, and craftmanship of the numerous methods of "directional drilling" have been applied, it is the purpose of this device, when knowledgeably used, to assist in the hole control with respect to both vertical and directional deviations and also to the straightness of the hole once the intentional deviations have been achieved.
Another type of stabilizer that has been in use is that of a non-rotating stabilizer. In general non-rotating stabilizers are more effective in controlling the hole angle due to the fact that rotatable tools may tend to drill the hole off course due to factors such as those mentioned above. With this type of stabilizer, the mandrel of the stabilizer rotates within the stabilizer member and must be lubricated. In the past, this has been accomplished by utilizing drilling fluid as a lubricant in combination with holes that are drilled in the member through which the fluid enters. In general the stabilizer sleeve is provided with a number of radially extending and circumferential spaced blade members. The furthest extension of the blades is generally seen to be that of the outer diameter of the drill bit, but generally not beyond that diameter. For example, when a 97/8" drilling bit is used to drill a well bore, and drill pipe stabilizers are installed in the drill string to limit deviation, the maximum diameter of the stabilizers is also 97/8" or the same as the bit.